1. Field of the Invention
The present invention relates to a carbon material to be used as a negative electrode of a lithium secondary battery and a method for producing the carbon material.
2. Description of the Related Art
Lithium secondary batteries are light weight and have high input/output characteristics compared with conventional secondary batteries such as nickel-cadmium batteries, nickel metal hydride batteries, and lead-acid batteries so that they have been considered promising in recent years as power sources for electric vehicles and hybrid vehicles. Such lithium secondary batteries usually comprises a lithium-containing positive electrode allowing reversible intercalation of lithium and a negative electrode comprising a carbon material. These electrodes are disposed opposite to each other via a non-aqueous electrolyte. Such batteries are therefore assembled in a discharged state so that they cannot discharge without charging. A charge/discharge reaction will next be described with, as an example, a lithium secondary battery comprising lithium cobaltate (LiCoO2) as a positive electrode, a carbon material as a negative electrode, and a lithium-salt-containing non-aqueous electrolyte solution as an electrolyte.
First, during charging of a first cycle, lithium contained in the positive electrode is released to the electrolyte solution (below-described Formula 1) and the positive electrode potential is shifted to the noble direction. At the negative electrode; lithium released from the positive electrode is occluded in the carbon material (below-described Formula 2) and the negative electrode potential is shifted to a less noble direction. Usually, when a difference between the positive electrode potential and the negative electrode potential, that is, battery voltage, reaches a predetermined value, charging is terminated. This voltage is called “charge termination voltage”. During discharging, lithium occluded in the negative electrode is released, the negative electrode potential is shifted to a noble direction, the lithium is occluded in the positive electrode again, and the positive electrode potential is shifted to a less noble direction. Discharging, similar to charging, is also terminated when a difference between the positive electrode potential and the negative electrode potential, that is, the battery voltage, reaches a predetermined value. This value is called “discharge termination voltage”. The whole reaction formula of charging and discharging as described above is represented by the following Formula 3. In cycles after the first cycle, the charge/discharge reaction (cycle) proceeds by the migration of lithium between the positive electrode and the negative electrode.

In general, carbon materials used as negative electrode materials in lithium secondary batteries are roughly classified into graphite-based ones and amorphous ones. Graphite-based carbon materials have an advantage of high energy density per unit volume compared to amorphous carbon materials. For this reason, graphite-based carbon materials are widely used as negative electrode materials in lithium ion secondary batteries for mobile phones and laptop computers that are compact but require large charge/discharge capacities. Graphite has a structure in which hexagonal network planes of carbon atoms have been stacked regularly one after another and during charging/discharging, intercalation/deintercalation of lithium ions takes place at the edges of the hexagonal network planes.
As described above, using lithium secondary batteries as an electric storage device for automobiles, industries, or power supply infrastructure has been studied briskly. When used for such purposes, they are required to have markedly high reliability compared with the case where they are used for mobile phones or laptop computers. The term “reliability” as used herein means a property related to product life, more specifically, a property not easily undergoing a change in charge/discharge capacity or internal resistance (i.e., not easily undergoing degradation) even when a charge/discharge cycle is repeated, even when the batteries are stored in charged state at a predetermined voltage, or even when they are charged continuously at a predetermined voltage (i.e., even when they are float-charged).
On the other hand, it is generally known that the life characteristics of lithium ion secondary batteries conventionally used for mobile phones or laptop computers largely depend on the material used as a negative electrode. The reason of it is because the charge/discharge efficiency in the positive electrode reaction (Formula 1) and the charge/discharge efficiency in the negative electrode reaction (Formula 2) cannot be made completely equal to each other in principle and the charge/discharge efficiency in the negative electrode is lower. The term “charge/discharge efficiency” as used herein means a ratio of an electric capacity which can be discharged to an electric capacity consumed for charging. A reaction mechanism causing deterioration in life characteristics due to the lower charge/discharge efficiency of the negative electrode reaction will hereinafter be described in detail.
During charging, as described above, lithium in the positive electrode is released (Formula 1) and occluded in the negative electrode (Formula 2). The electric capacity consumed for this charging is equal between the positive- and negative-electrode reactions. The charge/discharge efficiency is however lower in the negative electrode so that in the discharging reaction after the charging, discharging is terminated while a lithium amount released from the negative electrode is less than a lithium amount which can be occluded on the positive electrode, that is, a lithium amount which had been occluded on the positive electrode before the charging. The reason of it is because a part of the electric capacity which has been consumed at the negative electrode for charging is consumed for the side reaction and the competitive reaction and cannot be consumed for a lithium occlusion reaction to the positive electrode, that is, an occlusion reaction as a dischargeable capacity.
As a result of such a charging/discharging reaction, the positive electrode potential when discharge is terminated is shifted to a direction nobler than the potential before the charging/discharging, and the negative electrode potential is also shifted to a direction nobler than the potential before the charging/discharging. This occurs because of the following reasons. All lithium which has been released during charging of the positive electrode cannot be occluded back into or return to the positive electrode. Accordingly, during discharging, although a positive electrode potential which has been shifted to a noble direction during the charging before the discharging is shifted to a less noble direction, the potential cannot return to the original positive electrode potential by an amount corresponding to a difference in a charge/discharge efficiency between the positive electrode and the negative electrode. This leads to termination of the discharging at a potential more noble than the original positive electrode potential. As described above, discharging of a lithium secondary battery is completed at the time when a cell voltage, that is, a difference between positive electrode potential and negative electrode potential, reaches a predetermined value (discharge termination voltage). So, when the positive electrode potential is shifted to the noble direction upon discharge termination, the negative electrode potential will be also shifted similarly to the noble direction.
As described above, such lithium secondary batteries have a problem that when a charge/discharge cycle is repeated, an operation range of the capacity of the positive electrode and the negative electrode changes, resulting in degradation in capacity obtainable within a predetermined voltage range, or within a range of a discharge termination voltage and a charge termination voltage. Such a reaction mechanism of capacity degradation has already been reported in academic meetings or the like (for example, Proceedings of the 48th Battery Symposium in Japan, 1A11, Nov. 13, 2007, and Proceedings of the 76th Meeting of the Electrochemical Society of Japan, 1P29, Mar. 26, 2009). When once operation ranges of the positive/negative electrode potentials change, such changes are irreversible and the operation ranges of the positive/negative electrode potentials do not return to the original ones in principal. There is therefore no means for recovering the capacity, which has made the problem more serious.
The above-described reaction mechanism causing capacity degradation which occurs when the charge/discharge cycle is repeated is basically similar to a reaction mechanism which occurs when a battery is stored under a charged state or a reaction mechanism which occurs when a battery is float-charged. First, when a battery is stored under a charged state, it is known that a capacity lost by a side reaction and a competitive reaction which occur under a charged state (a self discharge amount) is greater in the negative electrode than in the positive electrode so that an operation range of the capacity of the positive/negative electrode changes between before and after storage and the battery capacity after storage decreases (for example, Proceedings of the 71st Meeting of the Electrochemical Society of Japan, 2I07, Mar. 24, 2004). A difference in self discharge rate between the positive and negative electrodes under a charged state owes to, similar to the above-mentioned difference in charge/discharge efficiency between the positive and negative electrodes, the fact that a side reaction or competitive reaction rate at the negative electrode under a charged state is higher than a side reaction or competitive reaction rate at the positive electrode under a charged state.
Next, when a battery is float-charged, both the positive electrode and negative electrode are charged respectively to have predetermined potentials continuously at the initial stage of charging. In fact, however, a current value (leakage current on the positive electrode side) necessary for keeping the positive electrode potential and a current value (leakage current on the negative electrode side) necessary for keeping the negative electrode potential is different. The reason of it is because as described above, self discharge rates under a charged state are different between the positive electrode and the negative electrode and the self discharge rate of the negative electrode is greater. At the time of float charging, a leakage current on the negative electrode side becomes greater than a leakage current on the positive electrode side so that a negative electrode potential is shifted to the decreasing direction of a leakage current, that is, the noble direction and a positive electrode potential is shifted to the increasing direction of a leakage current, that is, the noble direction. Thus, even if the battery is float-charged, an operation range of the capacity of the positive electrode and the negative electrode changes irreversibly, leading to degradation in battery capacity.